This application claims priority under 35 USC § 119 to Korean Patent Application No. 2004-52177, filed on Jul. 6, 2004, the contents of which are herein incorporated by reference in its entirety for all purposes.
1. Field of the Invention
The present invention relates generally to camera systems, and more particularly to a method and apparatus for controlling automatic exposure in a camera.
2. Description of the Related Art
Portable terminals, such as personal communication systems (PCS) and cellular phones, have become increasingly popular. Particularly, the portable terminal with a built-in camera, referred to as a camera phone, is widely used. In such a camera, an exposure time of each pixel needs to be controlled according to lighting conditions. When the exposure time is too long, an image may be overly bright. In contrast, when the exposure time is too short, an image may be too dark.
Conventional automatic exposure (AE) control systems adjust the exposure time of each pixel to compensate for variations in brightness of the image, which includes a subject and a surrounding (or background). The automatic exposure (AE) controls a shutter speed of the camera to maintain a brightness level of the captured image.
When the image is captured and converted to digital image data during digital image processing, an average brightness of the digital image data of each pixel is determined based on the brightness of the whole scene. The extracted average brightness is used as an input to the automatic exposure to control the exposure time so that the brightness of the image has a target value.
In conventional exposure control, an average brightness of the whole area of the captured image is used as the input to the automatic exposure. Therefore, when a light source such as the sun is positioned at a rear of the subject (back-lighted), the average brightness is relatively high despite the relatively large brightness difference between the subject and the background. Accordingly, the main subject may be under-exposed (i.e., the subject is too dark). When the light source is positioned at a front of the subject (front-lighted), the main subject may be over-exposed (i.e., the subject is too bright). Therefore, the conventional automatic exposure may not effectively compensate for such front-lighted or back-lighted scenes.
To compensate for the back-lighted or front-lighted scene, U.S. Patent Application Publication Number No. 2002-0136551 provides exposure control with comparison of brightness levels for a central area and a peripheral area of the captured image to determine whether the scene is back-lighted or front-lighted. The exposure control data is determined according to a logarithmic relationship with the brightness levels and with a correction coefficient that is determined according to a distance of the subject from the camera. In addition, such a correction coefficient is predefined and stored in memory.
Additionally, Japanese Patent Application Publication No. 2002-300468 discloses exposure control that divides the captured image into at least two areas to apply given weight values to the respective areas. When a ratio of a brightness of a predetermined area to a brightness of another area is greater than a given value, it is determined whether the scene is back-lighted so that the weight values applied to the respective areas are modified. However, a correction coefficient for each of the divided areas is predefined, and the brightness difference between the respective areas is not used in the exposure control.
Furthermore, power consumption is desired to be reduced in a mobile communication system such as a mobile phone or a personal digital assistant (PDA). In addition, simpler hardware is desired for reducing the thickness and size of the portable terminal.
Thus, an automatic exposure control amenable for mobile application devices is desired, particularly with accurate control of automatic exposure in the camera.